1. Field of the Invention
This invention relates to a water dispersible coating composition which comprises:
(i) an acrylic polymer having active hydrogen functionality and quaternary ammonium salt functionality; and PA1 (ii) a blocked polyisocyanate. PA1 (i) an acrylic polymer which comprises the free radical addition product of (a) at least one quaternary ammonium salt containing monomer having the structure: ##STR1## wherein R.sub.1 is H or alkyl of 1 to 3 carbons, Z is N--H or O; R.sub.2 is a divalent methylene radical --(CH.sub.2).sub.n -- wherein n is an integer from 1 to about 10; R.sub.3 and R.sub.4 can be the same or different and as alkyl or substituted alkyl, preferably of 1 to about 20 carbons or aryl or substituted aryl, preferably of 6 to about 18 carbons and R.sub.5 and R.sub.6 can be the same or different and are hydrogen, alkyl or substituted alkyl, preferably of 1 to about 20 carbons or aryl or substituted aryl, preferably of 6 to about 18 carbons; and X.sup.- is an anion; and (b) at least one active hydrogen functional unsaturated monomer; and (c) at least one other ethylenically unsaturated monomer; and PA1 (ii) a blocked polyisocyanate which is stable in the presence of the acrylic polymer at room temperature but reactive with the acrylic polymer at elevated temperatures. It is especially preferred to have R.sub.6 be hydrogen and R.sub.3, R.sub.4 and R.sub.5 be lower alkyl of 1 to 4 carbons. Substituted alkyl or aryl groups are those having one or more hydrogen atoms replaced with alkyl, aryl, aroxy, alkoxy or halogen groups and can contain acyl groups.
The water dispersible coating composition of this invention can be applied by any means known within the coatings art, but is especially suited for use in cathodic electrodeposition.
2. Description of the Prior Art
The use of quaternary ammonium salt containing polymers as water reducible coatings has been known in the art. For example, U.S. Pat. No. 3,935,087 teaches resins prepared by reacting a portion of the hydroxyl groups of a hydroxyl-containing epoxy group-containing organic material with a partially-capped or blocked organic polyisocyanate, followed by reacting at least a portion of the epoxy groups in a manner so as to add pendant quaternary onium salt groups to render the resin water dispersible or solubilized.
U.S. Pat. No. 4,071,428 teaches electrodepositable compositions which comprise the reaction product of a polymeric tertiary amine and a 1,2 epoxy containing material in the presence of acid and/or water to form quaternary ammonium base group-containing polymers. This approach has the drawback that the epoxy could also react with other functional groups on the polymer backbone, such as hydroxyl groups or primary and secondary amine groups.